The microtubule organizing centers (MTOC), the animal centrosome and its yeast functional homolog, the spindle pole body (SPB) originate and organize the cellular microtubule network and - in mitosis - the spindle that separates the sister chromatids from each other. The number of MTOCs is under precise control and their duplication that happens once and only once during a cell cycle is controlled by the cell cycle machinery, but these pathways are poorly understood. Abnormal centrosome numbers are often observed in cancer cells and abnormal centrosome duplication is thought to contribute to the development or even to the causation of cancer by giving rise to abnormal spindle formation and resulting chromosome missegregation resulting in aneuploidy. The present application proposes to study the duplication of the yeast SPB as a model of animal centrosome duplication. In the application two related genetic screens are proposed to define the pathways leading to SPB duplication. Both of the screens utilize a genetic assay to detect an increased rate of chromosome missegregation expected when SPB duplication is defective. In one approach a genetic screen will be performed to isolate mutants in the cell cycle kinase CDC28, that are defective in SPB duplication. Isolation of this mutant will lead to the possibility to define the form of the cell cycle kinase regulating SPB duplication and will be used to further define the pathway downstream from the cell cycle kinase. In the other approach an overexpression screen will be performed to identify genes that interfere with SPB duplication. From what is already known about the molecular composition and duplication of the SPB and the centrosome it seems likely, that what we learn about SPB duplication will be applicable to centrosome duplication.